Catalytic vapor phase oxidation reactor apparatus

ABSTRACT

A catalytic vapor phase oxidation reactor which comprises a fixed-bed shell and tube heat exchange apparatus in which a bundle of a multiplicity of tubes filled with at least one type of oxidizing catalyst are disposed in a shell and these tubes are passed through the apertures formed in at least one perforated shield plate to partition the inside of the shell into at least two heat transfer medium feed zones and in such a manner that each of the tubes passing through the perforated shield plate is not in direct contaction with the shield plate but the outer surface of the tube and the inner surface of the aperture are spaced apart by a distance of between 0.2-5 mm, supplying feed gas to the tubes of the reactor, and conducting exothermic catalytic vapor phase oxidation while controlling the temperatures for the heat transfer medium in each of the zones so that the temperature difference between each of the zones can be maintained between 0°-100° C.

This is a division of application Ser. No. 922,791, filed 7-7-78, nowU.S. Pat. No. 4,203,906, issued May 20, 1980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for catalytic vapor phase oxidationand a reactor used therefor and, in particular, to a process forcatalytically oxidizing hydrocarbons in vapor phase using a fixed bedmulti-tubular heat exchange type reactor. More specifically, it concernsa structure of a multi-tubular heat exchange type reactor which can keepa catalyst used therein at optimum reaction conditions and restrict thegeneration of hot spots (abnormal local heating in catalyst layers),upon exothermic catalytic vapor phase oxidation of hydrocarbons, and amethod of using such a reactor.

2. Discussion of the Relevant Art

Catalytic vapor phase oxidations are generally highly exothermic and itis thus very important to control the reaction temperature within acertain range and restrict the generation of hot spots in the reactionzones, which imposes great efforts on those skilled in the art. Nosatisfactory control of the catalytic reaction temperature with thecatalysts can be attained only with the uniform circulation of heattransfer medium in a reactor and hot spots appear frequently to resultin excess oxidation locally in the reactor, particularly where theoxidation reaction has to be proceeded sequentially to convert startingmaterials into end products such as in the oxidation of naphthalene oro-xylene into phthalic anhydride, oxidation of benzene, butylene orbutadiene into maleic anhydride, oxidation of propylene into acrolein oracrylic acid, oxidation of ethylene into ethylene oxide, ammoxidation ofpropylene into acrylonitrile, ammoxidation of aromatic hydrocarbons suchas toluene and xylene into aromatic nitriles sch as benzonitrile,phthalonitrile and the like. As the result undesired combustion reactionis increased to lower the yield of the aimed products. In addition,since the catalysts are always exposed locally to high temperature bythe presence of the hot spots, the life of the catalysts is decreased inthat portion to result in disadvantages.

Various counter measures have been employed in order to overcome theforegoing disadvantages in the vapor phase oxidation. As one of the mostpopular methods, the diameter of catalyst filled tube is decreased inorder to increase the heat transfer rate per unit volume of thecatalyst. This method is, however, defective in that the number of thefilled tube is increased and it increases the fabrication cost of thereactor, as well as takes much time for the charging and discharging ofthe catalyst.

In other effective methods proposed so far, catalyst layer is dilutedwith an inert substance or, as disclosed in Japanese PublishedUnexamined patent application No. 85485/1973, generation of hot spots isrestricted by the insertion of cylindrical material containing closedcavity in the center at the cross section of a reaction tube filled withthe catalyst entirely or partially in the axial direction of it therebyproviding a space in which no catalyst is present and no reactionmixture passes through. This method is, however, defective in that thecost is inevitably increased by so much as the substantially inertmaterial is contained. It has a further defect that recovery of usefulmetal components from the catalyst removed from the reactor after thedegradation of the catalytic activity is very laborious to lower therecovery efficiency.

A further effective method suppresses the temperature rise in the hotspots by gradually increasing the activity of the catalyst from theinlet to the outlet in the reaction tube. This method, however, requiresat least two types of catalysts of different catalytic activities, andno optimum reaction temperatures can be selected for respectivecatalysts charged in each of the layers. Moreover, if these catalystsshow different degrees of aging changes in their catalytic activities,control and keeping of the optimum reaction temperatures are furtherdifficult to inevitably lower the over all yield for the desiredproducts.

A still further effective method is proposed as disclosed in U.S. Pat.No. 3,147,084 and in German Laid Open Pat. Publication No. 2,513,405wherein a shell of a multi-tubular heat exchange type reactor isentirely partitioned with a shield plate into two heat transfer mediumfeed zones and reaction is carried out while circulating heat transfermedium at different temperatures in each of the zones. It is, however,very difficult in this method to insert as many as several thousands ofreaction tubes into a perforated plate used as the shield plate in thereactor, and those portions between the perforated plate and thereaction tubes that are contacted by the heat expansion of the tubes areabraded by the pulsation of the heat transfer medium to cause corrosionand destruction unless the reaction tubes and the perforated plate aresecured to each other by welding or expanding the diameter of thereaction tubes. The above securing fabrication however requirestroublesome works such as accurate perforation, welding and diameterexpansion over several thousands of portions.

It is, accordingly, an object of this invention to provide an improvedprocess of catalytic vapor phase oxidation and a reactor used therefor.

It is another object of this invention to provide a process of catalyticvapor phase oxidation in which catalyst is kept at its optimum reactionconditions and an apparatus used therefor.

It is a further object of this invention to provide a structure of amulti-tubular heat exchange type reactor capable of restricting thegeneration of hot spots and a method of using such a reactor.

These objects of this invention can be attained by the catalytic vaporphase oxidation process which comprises using a fixed-bed shell and tubeheat exchange type reactor in which a bundle of multiplicity of tubesfilled with at least one type of oxidation catalyst is disposed in ashell and these tubes are passed through the apertures perforation in atleast one perforated shield plate to partition the shell into at leasttwo heat transfer medium feed zones in such a way that each of the tubespassed through the perforated shield plate is not in direct contactionwith the shield plate but the outer surface of the tubes and the innersurface of the apertures are spaced apart with a distance between 0.2-5mm, feeding feed gas to the tubes of the reactor, and conducting theexothermic catalytic vapor phase oxidation while controlling thetemperatures of the heat transfer medium in each of the zonespartitioned by the shield plate so that the temperature differencetherebetween is kept within the range between 0°-100° C.

In order to obtain high yield an improved reactor for varying reactiontemperatures corresponding to reaction stages has hitherto been proposed(Japanese Published Unexamined patent application No. 80473/1973.)However, settlement of the temperature described in the patentapplication No. 80473/1973 is to carry out the reaction smoothly byproviding a controller in a circulation mechanism of the heat transfermedium, so it is difficult to obtain relatively isolated reactiontemperature zones as in the present invention. In the above process itis rather proposed to provide reaction zones wherein the heat transfermediums are completely separated from each other as a means forobtaining such shielded reaction temperature zones. Therefore, it isclear that the present invention relates to a simple and economicalreactor having a novel construction.

The reactor specified hereinbefore for use in the process of the presentinvention has an advantage in that the structure is simple to make, hasreduced cost and, in addition, as detailed hereinafter, it is notdistorted by heat. The use of this reactor in catalytic vapor phaseoxidation enables one to control the temperature of the heat transfermedium in the catalyst layer region where the exotherm is mostsignificant to a lower level than the temperature for the heat transfermedium in the other region thereby restricting the exotherm in the hotspots. This enables one to increase the conversion rate of the feed gasto be oxidized in the succeeding zone substantially to 100% and thuspermits most effective utilization of the catalyst. Particularly, theuse of the above reactor according to this invention in the catalyticvapor phase reaction where the reaction is proceeded sequentially,restricts useless combustion caused by the over oxidation in the hotspots to ensure improved yield in the desired products and enables toincrease the concentration of the starting material as compared withthat in the conventional catalytic vapor phase reaction. The reactionwhere catalysts and reaction temperatures are different in each of thereaction steps and, hence, two or more reactors have been required sofar can be conducted with only one reactor by the process according tothis invention. Moreover, this invention provides a further merit thatthe catalyst life is prolonged astonishingly.

BRIEF DESCRIPTION OF DRAWINGS

This invention will be understood best in connection with theaccompanying drawings, wherein:

FIG. 1 is a vertical cross sectional view of a reactor for carrying outthe process of this invention;

FIG. 2 is a cross sectional view of a part taken along line II--II inFIG. 1;

FIG. 3 is a cross sectional view of a part of the shield plate mountingportion in another embodiment of this invention;

FIG. 4 is a cross sectional view of a part of the shield plate mountingportion in still another embodiment of this invention; and

FIG. 5 is a cross sectional view of a part of the shield plate mountingportion in a still further embodiment of this invention.

FIG. 1 shows one example of a fixed-bed type shell and tube heatexchange type reactor 1, whose shell 2 contains in its inside a lot of(for example several hundreds to several thousands or more) reactiontubes in a diameter, for example, of 5-50 mm loaded in parallel with theaxis of the shell 2. Each of the reaction tubes 3 is fitted at its upperand the lower end respectively to each of the apertures in tube plates 4and 5 respectively and secured thereto by means of diameter expansionand welding. At the lower portion of the reaction tubes, are secured awire mesh screen 6 for the prevention of catalyst falling and aperforated plate 7 to the shell 2 by means of welding and the like. Theshell 2 is secured at its upper and the lower ends with a front cap 8and a rear cap 9 by means of welding or the like. The inside of theshell 2 is divided into at least two heat transfer medium zones A and Bby the disposition of at least one perforated plate 10 at a desiredposition. Plate 10 is provided with apertures 11 through which arepassed the reaction tubes 3. As shown in FIG. 1 and FIG. 2, theperforated shield plate 10 is not in direct contaction with each of thereaction tubes 3 passing therethrough but the outer surface of thereaction tube 3 and the inner surface of the apertures 11 are spacedapart at a distance between 0.2-5 mm. The provision of the distance isimportant. If the reaction tubes 3 and the shield plate 10 are arrangedclosed or secured to each other with no gaps, the reaction tubes 3 andthe shield plate 10 are undesirably contacted to each other and abraseddue to the heat distortion resulted in the reaction tubes 3 or theshield plate 10 because of the temperature difference between the zonesA and B, or frequent heating or cooling effected in the reactor.Moreover, the fabrication of the reactor is laborious and costly. On thecontrary, too wide gap makes the temperature control more difficult asthe amount of heat medium moving between the zones A and B is increased.It is required, based on our experience, that the distance between theouter surface of the reaction tube 3 and the inner surface of theapertures 11 in the shield plate 10 is between 0.2-5 mm and, preferably,0.3-1 mm for performing satisfactory temperature control with nosubstantial movement of the heat transfer medium between the zones A andB.

Heat transfer medium for heat exchange is supplied to the outer side 12of the reaction tube bundle (shell side) in the reactor 1 for keepingthe reaction temperature constant in the reaction tubes and it isintroduced for heat exchange by way of a volute pump or an exial flowpump 13 and 14 from inlets 15 and 16 and through annular conduits 17 and18 into the zones A and B respectively. Then, it is discharged throughannular conduits 19 and 20 and from exits 21 and 22 respectively andsent to heat exchangers (or heating devices) 25 and 26 for cooling (orheating) and then further circulated.

The method of circulating the heat transfer medium is noway limited onlyto the foregoing method. If the temperature difference is very greatbetween the zones A and B, keeping and control of the reactiontemperature can be facilitated by making lateral flow directions of theheat transfer medium at the shield plate 10 identical by introducing theheat medium into the outlet 22 in the zone B where the heat medium isintroduced into the inlet 15 in the zone A. It is also possible toprovide a flow control mechanism to each of the circulating mechanismsfor facilitating the temperature control in each of the temperaturezones, and either one of the heat transfer medium cooling (or heating)means 25 and 26 can be saved where exotherm (or endotherm) in the zonesA and B, the moving amount of the heat transfer medium between the zonesA and B and the circulating amount of the heat transfer medium by way ofthe circulating devices of the pumps 13 and 14 are known previously.

The number of the shield plates 10 may be increased to more than onewhere more strict temperature control is required for both of the zonesA and B. It is recommended, where the reactor has a great diameter andthus a lot of reaction tubes, to change the direction of flow of theheat transfer medium by baffle plates 27 to thereby increase the heatexchange efficiency.

At least one type of catalyst in the form of granule such as inspherical, pellet and irregular form is charged in the reaction tube 3and the feed gas is supplied through the conduit 28 to the reactor 1 andthe gas contacts the catalyst in the reaction tube 3 to conduct theoxidation reaction. The reaction heat generated in the course of thereaction is heat exchanged with the heat transfer medium to keep thecatalyst layer at a predetermined temperature. The reaction mixturecontaining the desired products is sent through the conduit 29 tocollection, recovery and purification steps. In the above reactionsteps, the starting reaction material may be alternatively introducedfrom the conduit 29, passed through the reaction tube 3 and thendischarged out of the conduit 28.

FIG. 3 shows another embodiment of this invention and it shows a sectionin elevation of a part of a shield plate 30 in which no substantialmovement of the heat transfer medium takes place between the zones A andB because of annular fins 34 secured to reaction tubes 33. This preventsthe heat transfer medium from moving by keeping the distance between theouter surface of the reaction tube 33 and the inner surface of theapertures 31 in the shield plate 30 to between 0.2-5 mm. The presence ofthe distance is important and, if the reaction tube 33 and the shieldplate 30 are too closely spaced or secured to each other, the reactiontube and the shield plate are undesirably contacted and abraded wheretemperature difference between the zones A and B is great or frequentheating or cooling takes place in the reactor. Moreover, fabrication ofthe reactor is laborious. On the contrary, it is not necessary anddisadvantageous to make the gap excessively wide. Since the distancebetween the reaction tube and another is selected between 6-30 mm in ausual multi-tubular reactor, the distance between the shield plate andthe reaction tube is naturally restricted thereby.

The fin is secured to the reaction tube in such a manner as it coversthe distance described above as shown in FIG. 3. The distance betweenthe fin and the shield plate is controlled to between 0.2-5 mm and,preferably, 0.3-1 mm, whereby no substantial movement of the heattransfer medium between zones A and B is effected in its circulation andthe temperature for each of the reaction temperature zones can becontrolled with satisfaction.

The fin may be disposed in parallel with the shield plate or it may beconnically-shaped and secured to the reaction tube 43 in such a mannerthat the top end of the fin 44 comes nearest to the tube and extendsoutwardly towards the shield plate as shown in FIG. 4. The fin 44 may beattached either above or below the shield plate 40, or as shown in FIG.5 fins 54 may be attached to the tubes 53 so that they alternately aredisposed above and below the shield plate 50.

The distance above described as between 0.2-5 mm, preferably, 0.3-1 mmis somewhat influenced by the type of the heat transfer medium used.When highly viscous medium, for example, molten salt (mainly composed ofa mixture of potassium nitrate and sodium nitrite) is used, the reactiontemperature is high and the amount of the heat transfer medium passingthrough the gap can be small even if the distance is somewhat wide. Inusing other heat transfer medium such as phenyl ether medium (forexample "Dowtherm") and polyphenyl medium (for example "Therm S"), itis, however, desired to make the distance somewhat narrower even in alower temperature reaction as compared with the use of the molten salts.

The heat transfer medium used in this invention include, in addition tothe above medium, hot oil, naphthalene derivatives (S.K. oil), mercuryand the like.

While the process of this invention can be applied to any exothermiccatalytic vapor phase oxidation, it is particularly advantageous for thecatalytic vapor phase oxidation of hydrocarbons including variousproduction processes such as oxidation of naphthalene or o-xylene intophthalic anhydride, oxidation of benzene, butylene or butadiene intomaleic anhydride oxidation of propylene into acrolein or acrylic acid,oxidation of ethylene into ethylene oxide, ammoxidation of propyleneinto acrylonitrile, oxidation of isobutylene into methacrolein ormethacrylic acid, ammoxidation of isobutylene into methacrylonitrile,ammoxidation of aromatic hydrocarbons such as toluene and xylene intoaromatic nitriles such as benzonitrile and phthalonitrile, oxidation ofnaphthalene into naphthoquinone, and oxidation of anthracene intoanthraquinone. In these reactions, hydrocarbons and molecular oxygen areintroduced with the co-existence of an inert gas if required into thereactor and oxidized into a desired product.

The catalytic vapor phase oxidation can thus be effected with an extremeease by using the reactor specified in this invention. As stated above,while the reactor defined by the present invention is best suited to theconduction of the sequential oxidation, it has been found based on ourexperience that the temperature difference as great as 100° C. at themaximum can be set between the heat transfer medium in each of thezones. The temperature difference between the heat transfer medium ineach of the zones is therefore between 0°-100° C. and, preferably,0°-80° C. As for the reason of necessity of the temperature differenceof 0° C. specified in above, there is a case that two or more sequentialreactions having highly different heats of reaction from each other canbe advantageously carried out at nearly the same temperature only bycontrolling the flow rates of the heat transfer medium in each zone inthe reactor of this invention. Further, it means that at the beginningof the reaction, even if several tens degree centigrade of temperaturedifference is required, activities of the catalyst varies gradually withthe lapse of time and the temperature difference between each zonedecreases, and finally the temperature difference is sometimes reversed.

More specifically, in the production of phthalic anhydride from o-xyleneor naphthalene using two reaction temperature zones A and B, temperaturebetween 300°-400° C. is employed in the preceeding stage and thetemperature between 350°-450° C. is employed in the subsequent stage andthe temperature difference has to be kept between 30°-60° C. if thecatalyst of a same composition is employed. Such a condition can besatisfied with ease. In the oxidation of benzene, butylene or butadieneinto maleic anhydride using two reaction temperature zones A and B, thetemperature for the preceeding stage is at 320°-400° C. and thetemperature for the subsequent stage is at 350°-450° C. and it isrequired to maintain the temperature difference between 20°-50° C. Theabove condition can also be satisfied with ease.

Satisfactory results can also be obtained in carrying out the process ofthis invention using catalysts of two or more different compositions,because the reaction can be proceeded at reaction temperatures moresuited to the performance of the respective catalysts.

It has been found, more characteristically to this invention, that theprocess according to this invention is also applicable to a catalytictwo phase oxidation process as comprising a preceeding oxidation ofpropylene into acrolein and a succeeding oxidation of propylene intoacrolein and a succeeding oxidation of the acrolein into acrylic acidwhere the reaction temperature in each of the reaction zones isdifferent as much as by 50°-100° C.

Example 1

O-xylene was catalytically oxidized by air in vapor phase into phthalicanhydride using a vertical type multi-tubular reactor as shown in FIG. 1having 24 steel tubes 4 m in length, 25.0 mm in inside diameter and 29.0mm in outside diameter, in which a shield plate is situated at the halfheight of the reactor and the distance between the reaction tubespassing through the shield plate and the shield plate was adjusted toabout 0.6 mm. The catalyst employed in this oxidation reaction wasprepared in accordance with the description of Example 1 in U.S. Pat.No. 3,926,846 and had a catalyst composition: V₂ O₅ :TiO₂ =2.1:97.9 onthe weight basis and, based on the total weight of V₂ O₅ and TiO₂, 0.49%by weight of P₂ O₅, 0.146% by weight of K₂ O and 0.25% by weight of Nb₂O₅. The porosity of the catalyst was measured by mercury porosimeter,and the pore volume of pore diameters of 0.10-0.45 micron amounted to86% of the total volume of pores of diameters of less than 10 microns.

The catalyst thus prepared was charged by 1,500 cc per one reaction tubeso as to give a 3 m catalyst layer length, 1 m of the total layer lengthsituating a temperature zone in the preceding Stage (A) and remaining 2m situating in the temperature zone in the subsequent stage (B).

In the initial stage of the reaction, the temperature of the molten salton the shell side of the reactor was maintained at 355°C. in thetemperature zone A and at 375°C. in the temperature zone B, and thereaction was started at a concentration of 20(l) air/o-xylene(g) and ata space velocity (S.V.) of 4,000 hr⁻¹. The reaction was continuedthereafter for one year while controlling the temperature in both of thetemperature zones A and B so that the optimum yield was obtained forphthalic anhydride. The result are shown in Table 1. In the Table 1, theyield for phthalic anhydride is expressed by weight % basis on o-xylenesupplied. The gas concentration (G.C.) is for the concentration air(l)/o-xylene (g).

                  TABLE 1                                                         ______________________________________                                               Reaction                   Phthalic                                           temperature (°C.)                                                                  S.V.    G.C.   anhydride                                   Time elapsed                                                                           A        B        (hr.sup.-1)                                                                         (l/g)                                                                              yield (wt. %)                           ______________________________________                                        Initial  355      375      4,000 20   115.3                                    3 month 355      375      4,000 20   115.0                                    6 month 357      375      4,000 20   114.5                                   12 month 359      375      4,000 20   114.1                                   ______________________________________                                    

COMPARATIVE EXAMPLE 1

Reaction was continued for 12 months while using the same catalyst as inExample 1, using a same scale of a reactor as in Example 1 where noshield plate is provided so as to form a single temperature zone andunder the reaction conditions shown in Table 2. The results are alsoshown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                            Phthalic                                           Reaction      S.V.    G.C. anhydride                                 Time elapsed                                                                           temperature (°C.)                                                                    (hr.sup.-1)                                                                           (l/g)                                                                              yield (wt. %)                             ______________________________________                                        Initial  370           4,000   20   112.8                                      3 month 375           4,000   20   112.1                                      6 month 381           4,000   20   110.4                                     12 month 390           4,000   20   105.9                                     ______________________________________                                    

EXAMPLE 2

Reaction was conducted for 12 months using the same catalyst and thesame reactor as those in Example 1, increasing the gas concentration to16 (l) air/o-xylene (g) and under the reaction conditions as shown inTable 3. The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                               Reaction                   Phthalic                                           temperature (°C.)                                                                  S.V.    G.C.   anhydride                                   Time elapsed                                                                           A        B        (hr.sup.-1)                                                                         (l/g)                                                                              yield (wt. %)                           ______________________________________                                        Initial  360      380      4,000 16   114.5                                    3 month 363      380      4,000 16   113.9                                    6 month 365      383      4,000 16   113.5                                   12 month 368      389      4,000 16   113.0                                   ______________________________________                                    

COMPARATIVE EXAMPLE 2

Reaction was continued for 3 months increasing the gas concentration inComparative Example 1 to 16 (l) air/o-xylene (g) and under the reactionconditions shown in Table 4. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                                            Phthalic                                           Reaction      S.V.    G.C. anhydride                                 Time elapsed                                                                           temperature (°C.)                                                                    (hr.sup.-1)                                                                           (l/g)                                                                              yield (wt. %)                             ______________________________________                                        Initial  380           4,000   16   104.1                                     3 month  407           4,000   16    97.7                                     ______________________________________                                    

EXAMPLE 3

Phthalic anhydride was prepared according to the process in Example 1and using two types of catalysts. The catalysts were prepared accordingto the descriptions of Example 1 in U.S. Pat. No. 4,046,780. A catalysthaving a catalytically active substance of a composition:

V₂ O₅ :TiO₂ :Nb₂ O₅ :P₂ O₅ :K₂ O:Na₂ O =2:98:0.25:1.02:0.15:0.1 (weightbase) was prepared as the catalyst for the preceding stage. The porositydistribution of the catalyst was measured by a mercury porosimeter. Thepore volume of pore diameters of 0.15-0.45 micron amounted to 88% of thetotal pore volume of pores of diameters of less than 10 microns, whichis to be referred to as 88% pore volume of 0.15-0.45 micron hereinafter.The catalyst was used as the preceding stage catalyst.

Then, another catalyst having a catalytically active substance of acomposition:

V₂ O₅ :TiO₂ :Nb₂ O₅ :P₂ O₅ :K₂ O:Na₂ O =2:98:0.25:1.3:0.15:0.1 (weightbasis) and having 87% pore volume of 0.15-0.45 micron was prepared asthe subsequent stage catalyst.

The catalysts thus prepared were charged in the reaction tubes in thesame reactor as employed in Example 1 in which the subsequent stagecatalyst was filled to a length of 1.5 m in the temperature zone B andthen the preceding zone catalyst was filled to a length of 1.5 m in thetemperature zone A and the reaction was effected. The reactionconditions and the results of the reaction are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                               Reaction                   Phthalic                                           temperature (°C.)                                                                  S.V.    G.C.   anhydride                                   Time elapsed                                                                           A        B        (hr.sup.-1)                                                                         (l/g)                                                                              yield (wt. %)                           ______________________________________                                        Initial  360      375      3,500 16.6 117.2                                   3 month  365      375      3,500 16.6 117.0                                   6 month  370      375      3,500 16.6 116.9                                   ______________________________________                                    

EXAMPLE 4

Maleic anhydride was obtained from benzene in the same reactor as inExample 1. The catalyst used in this oxidation was prepared according tothe descriptions of Example 1 in U.S. Pat. No. 4,036,780. The completedcatalyst prepared had a catalytically active substances of a composition:

V₂ O₅ :MoO₃ :P₂ O₅ :Na₂ O =1:0.40 : 0.015: 0.06 (molar ratio).

The catalyst thus prepared was charged by 1,500 cc per one reaction tubeso as to give a 3 m catalyst layer length.

In the initial stage of the reaction, temperature was maintained at 345°C. for the temperature zone A and at 370° C. for the temperature zone B,and the reaction was started at a gas concentration of 22 (l)air/benzene (g) and at a space velocity of 2500 hr⁻¹ (NTP). The reactionwas continued thereafter for 12 months while controlling thetemperatures for both of the zones A and B so that the best yield wasobtained for the yield of maleic anhydride. The results are shown inTable 6.

                  TABLE 6                                                         ______________________________________                                               Reaction                   Maleic                                             temperature (°C.)                                                                  S.V.    G.C.   anhydride                                   Time elapsed                                                                           A        B        (hr.sup.-1)                                                                         (l/g)                                                                              Yield (wt. %)                           ______________________________________                                        Initial  345      370      2,500 22   94.2                                     3 month 347      370      2,500 22   92.8                                     6 month 350      372      2,500 22   93.0                                    12 month 355      375      2,500 22   92.7                                    ______________________________________                                    

COMPARATIVE EXAMPLE 3

Reaction was continued using the same catalyst as employed in Example 4and employing a same scale of a reactor as used in Example 4, in whichno shield plate was provided so as to form a single temperature zone andunder the reaction conditions shown in Table 7. The results are shown inTable 7.

                  TABLE 7                                                         ______________________________________                                                                            Maleic                                             Reaction      S.V.    G.C. anhydride                                 Time elapsed                                                                           temperature (°C.)                                                                    (hr.sup.-1)                                                                           (l/g)                                                                              yield (wt. %)                             ______________________________________                                        Initial  370           2,500   22   93.2                                       3 month 378           2,500   22   90.0                                       6 month 385           2,500   22   85.3                                      12 month 390           2,500   22   81.7                                      ______________________________________                                    

EXAMPLE 5

Acrylic acid was obtained by the oxidation of propylene in a similarreactor to that in Example 1 excepting that the length of a reactiontube was 6 m. As the catalysts used in this oxidation, a preceding stagecatalyst for mainly preparing acrolein from propylene was preparedaccording to the descriptions of Example 1 in U.S. Pat. No. 3,825,600and the subsequent stage catalyst for oxidizing the acrolein intoacrylic acid was prepared according to the descriptions of Example 1 inU.S. Pat. No. 3,833,649. The preceding stage catalyst was an oxidizingcatalyst having a composition, except for oxygen, of: Co₄ Fe₁ Bi₁ W₂Mo₁₀ Si₁.35 K₀.06 in atomic ratio, and the subsequent stage catalyst wasan oxidizing catalyst supported on a support and having a metalcomposition of : Mo₁₂ V₄.6 Cu₂.2 Cr₀.6 W₂.4.

The subsequent stage catalyst was at first charged each by 1,250 cc perone reaction tube in the temperature zone B to form a layer height of2.5 m. Then, 250 cc of 5 mm of diameter of spheric alundum was filledthereover for cooling reaction gas in such a manner that its upper endlevelled with the plane of the shield plate. Then, the preceding stagecatalyst was charged further thereover so as to form a 2.4 m chargedlayer length. A gas mixture having a reaction gas composition of 7.0% byvolume of propylene, 12.6% by volume of oxygen, 10.0% by volume of steamand balance of inert gas mainly containing nitrogen was supplied to thepreceding stage catalyst at a space velocity (S.V.) of 1,350 hr⁻¹ (NTP)and the reaction was started while maintaining the temperature at 320°C. for the temperature zone A and at 255° C. for the temperature zone Bat the initial stage of the reaction. The reaction was continuedthereafter for 12 months while controlling the temperature in both ofthe temperature zones A and B so as to obtain the optimum yield foracrylic acid. The results are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                               Reaction    S.V. to          Acrylic acid                                     temperature preceeding       single                                    Time   (°C.)                                                                              catalyst  G.C.   pass yield                                elapsed                                                                              A       B       (hr.sup.-1)                                                                           (mol %)                                                                              (mol %)                                 ______________________________________                                        Initial                                                                              320     255     1,350   7      86.5                                     3 month                                                                             320     260     1,350   7      86.9                                     6 month                                                                             320     266     1,350   7      87.5                                    12 month                                                                             320     270     1,350   7      88.0                                    ______________________________________                                    

Having thus set forth the nature of the invention, what is claimed is:
 1. A fixed-bed shell and tube heat exchange type reactor for use in exothermic catalytic vapor phase oxidation, comprising: a bundle of a multiplicity of tubes filled with at least one type of oxidizing catalyst disposed in said shell, said tubes being passed through apertures provided in at least one perforated shield plate to partition the inside of said shell into at least two heat transfer medium feed zones in such a manner that each of said tubes passing through said perforated shield plate is not in direct contact with said shield plate, the outer surface of said tube and the inner surface of the apertures provided in said shield plate are spaced apart by a distance of between 0.2-5 mm.
 2. A reactor according to claim 1, wherein each of said heat transfer medium feed zones is respectively provided with means for circulating the heat transfer medium.
 3. A reactor according to claim 2, wherein only one shield plate is provided on the inside of said shell partitioning said shell into two heat transfer medium feed zones.
 4. A reactor according to claim 2, wherein the gap between the outer surface of said tubes and the inner surface of the aperture in the perforated shield plate is between 0.3-1 mm.
 5. A reactor according to claim 2, wherein said tubes include fins having an outer diameter of a size capable of covering the gap between the outer surface of said tube and the inner surface of said aperture in said perforated shield plate, said fins being individually secured to said tubes in the vicinity of said shield plate.
 6. A reactor according to claim 5, wherein the annular fins are flat and are generally parallel to said shield plate.
 7. A reactor according to claim 5, wherein said annular fins are provided only on one side of said shield plate.
 8. A reactor according to claim 5, wherein said annular fins are provided alternately on both sides of said shield plate.
 9. A reactor according to claim 5, wherein said annular fins are generally conically-shaped and disposed so as to outwardly open toward the gap between said tube and said shield late. 